Silicon-based image sensors provide high sensitivity in the visible portion of the optical spectrum. Improvements in CMOS sensors have provided the basis for low light detection in the visible portion of the spectrum. However, under night viewing conditions, a significant portion of the energy is at wavelengths greater than 1100 nm, which is the longest wavelength that is detectable in a silicon imaging array. Hence, an imaging system that is to operate under nighttime conditions requires a different material system to detect the energy in the near infrared (NIR) portion of the spectrum.
Material systems that can detect NIR photons are known to the art. For example, quantum dot (QD) materials convert NIR photons into electrons that can be collected and measured using CMOS circuitry. QD materials have bandgaps that can be tuned by adjusting the dot size. Such materials have been suggested for use in multi-junction solar cells to improve the efficiency of the cells.
An image sensor that can operate both in the NIR and the visible would be advantageous for forming images both under daylight and nighttime viewing conditions. Since QD based photodetectors rely on different material systems from conventional CMOS imaging detectors, providing such a hybrid imaging sensor presents significant challenges.